Refractive index changes in amorphous SiO2 (silica) by swift ion irradiation

The refractive index changes induced by swift ion-beam irradiation in silica have been measured either by spectroscopic ellipsometry or through the effective indices of the optical modes propagating through the irradiated structure. The optical response has been analyzed by considering an effective homogeneous medium to simulate the nanostructured irradiated system consisting of cylindrical tracks, associated to the ion impacts, embedded into a virgin material. The role of both, irradiation fluence and stopping power, has been investigated. Above a certain electronic stopping power threshold (∼2.5 keV/nm), every ion impact creates an axial region around the trajectory with a fixed refractive index (around n = 1.475) corresponding to a certain structural phase that is independent of stopping power. The results have been compared with previous data measured by means of infrared spectroscopy and small-angle X-ray scattering; possible mechanisms and theoretical models are discussed.

Kinetics of color center formation in silica irradiated with swift heavy ions: Thresholding and formation efficiency

We have determined the cross-section σ for color center generation under single Br ion impacts on amorphous SiO2. The evolution of the cross-sections, σ(E) and σ(Se), show an initial flat stage that we associate to atomic collision mechanisms. Above a certain threshold value (Se > 2 keV/nm), roughly coinciding with that reported for the onset of macroscopic disorder (compaction), σ shows a marked increase due to electronic processes. In this regime, a energetic cost of around 7.5 keV is necessary to create a non bridging oxygen hole center-E′ (NBOHC/E′) pair, whatever the input energy. The data appear consistent with a non-radiative decay of self-trapped excitons.

Tailoring the Optical Properties of Silica Irradiated with Swift Heavy Ions

Irradiation with swift heavy ions (SHI), roughly defined as those having atomic masses larger than 15 and energies exceeding 1 MeV/amu, may lead to significant modification of the irradiated material in a nanometric region around the (straight) ion trajectory (i.e., latent tracks). In the case of amorphous silica it has been reported that SHI irradiation originates nano-tracks of either higher density than the virgin material (for low electronic stopping powers, Se < 7 keV/nm) [1] or having a low-density core and a dense shell (Se > 12 keV/nm) [2]. The intermediate region has not been studied in detail but we will show in this work that essentially no changes in density occur in this zone. An interesting effect of the compaction is that the refractive index is increased with respect to that of the surroundings. In the first Se region it is clear that track overlapping leads to continuous amorphous layers that present a significant contrast with respect to the pristine substrate and this has been used to produce optical waveguides. The optical effects of intermediate and high stopping powers, on the other hand, are largely unknown so far. In this work we have studied theoretically (molecular dynamics and optical simulations) and experimentally (irradiation with SHI and optical characterization) the dependence of the macroscopic optical properties (i.e., the refractive index of the effective medium, n_EMA) on the electronic stopping power of the incoming ions. Our results show that the refractive index of the irradiated silica is not increased in the intermediate region, as expected; however, the core-shell tracks of the high-Se region produce a quite effective enhancement of n_EMA that could prove attractive for the fabrication of optical waveguides at ultralow fluences (as low as 1E11 cm^-2). 1. J. Manzano, J. Olivares, F. Agulló-López, M. L. Crespillo, A. Moroño, and E. Hodgson, "Optical waveguides obtained by swift-ion irradiation on silica (a-SiO2)," Nucl. Instrum. Meth. B 268, 3147-3150 (2010). 2. P. Kluth, C. S. Schnohr, O. H. Pakarinen, F. Djurabekova, D. J. Sprouster, R. Giulian, M. C. Ridgway, A. P. Byrne, C. Trautmann, D. J. Cookson, K. Nordlund, and M. Toulemonde, "Fine structure in swift heavy ion tracks in amorphous SiO2," Phys. Rev. Lett. 101, 175503 (2008).

In-situ optical reflectance characterization of ion beam irradiation damage on crystalline (quartz) and amorphous (silica) SiO2

Outline: • Motivation, aim • Complement waveguide data on silica • Optical data in quartz • Detailed analysis, i.e. both fluence kinetics and resolution • Efficiency of irradiation and analysis, samples, time... • Experimental set-up description • Reflectance procedure • Options: light source (lasers, white light..), detectors, configurations • Results and discussion • Comparative of amorphous and crystalline phases

Ta2O5/SiO2 insulating acoustic mirrors for AlN-based X-band BAW resonators

This work describes the performance of AlN-based bulk acoustic wave resonators built on top of insulating acoustic reflectors and operating at around 8 GHz. The acoustic reflectors are composed of alternate layers of amorphous Ta2O5and SiO2 deposited at room temperature by pulsed-DC reactive sputtering in Ar/O2 atmospheres. SiO2 layers have a porous structure that provides a low acoustic impedance of only 9.5 MRayl. Ta2O5 films exhibit an acoustic impedance of around 39.5 MRayl that was assessed by the picoseconds acoustic technique These values allow to design acoustic mirrors with transmission coefficients in the centre of the band lower than -40 dB (99.998 % of reflectance) with only seven layers. The resonators were fabricated by depositing a very thin AlN film onto an iridium bottom electrode 180 nm-thick and by using Ir or Mo layers as top electrode. Resonators with effective electromechanical coupling factors of 5.7% and quality factors at the antiresonant frequency around 600 are achieved.

Electronic damage in quartz (c-SiO2) by MeV ion irradiations: Potentiality for optical waveguiding applications

The damage induced on quartz (c-SiO2) by heavy ions (F, O, Br) at MeV energies, where electronic stopping is dominant, has been investigated by RBS/C and optical methods. The two techniques indicate the formation of amorphous layers with an isotropic refractive index (n = 1.475) at fluences around 1014 cm−2 that are associated to electronic mechanisms. The kinetics of the process can be described as the superposition of linear (possibly initial Poisson curve) and sigmoidal (Avrami-type) contributions. The coexistence of the two kinetic regimes may be associated to the differential roles of the amorphous track cores and preamorphous halos. By using ions and energies whose maximum stopping power lies inside the crystal (O at 13 MeV, F at 15 MeV and F at 30 MeV) buried amorphous layer are formed and optical waveguides at the sample surface have been generated.